Carbon-carbon composites are the material of choice for friction materials used in automotive and aerospace applications. One of the most important properties of brake discs in general, including brake discs that are suitable for use as components in the landing systems of large aircraft, is their friction and wear characteristics. While other factors, such as their weight, strength, durability and cost are also important, it is the friction and wear performance that controls the smooth performance and safety of the brakes on the aircraft. C—C composites provide good strength, friction performance, thermal properties and have the advantage of light weight over metallic friction materials.
To date, many different processing sequences have been developed to effect manufacture of carbon-carbon composite brake discs. CVI (chemical vapor infiltration) and CVD (chemical vapor deposition) are two essentially interchangeable processing techniques that are well known to persons skilled in the art of manufacturing carbon-carbon composites. Traditionally, carbon-carbon composites have been made by combining carbon fibers (PAN or pitch) with CVI/CVD and/or resin matrices. For purposes of convenience, this application will often refer to “CVD” alone. Persons skilled in the art will recognize that “CVI” processing could be used in place of any such disclosed CVD processing in the present invention.
Typically multiple cycles of CVI/CVD are required to fully densify the C—C composite to achieve a final density of >1.7 g/cc. Recently, pitch matrix C—C composites have also been used either alone or in combination with CVD/CVI and/or resin matrices for the manufacture of C—C Composites for braking applications. For instance, US 2006/0177663 A1 (Simpson et al.) discloses a carbon-carbon composite article such as a brake disc. Example 3 in this published application makes a preform from carbonized fibers which is infiltrated with coal tar pitch which is carbonized, followed by densification by resin transfer molding (RTM) processing (with high carbon yielding synthetic meso pitch) and/or CVI/CVD processing, providing a preform having a density of 1.75 g/cc. U.S. Pat. No. 6,537,470 B1 (Wood et al.) discloses rapid densification of preforms using RTM. This patent teaches RTM densification of a CVD rigidized preform and optional additional treatments of the densified part, including carbonization and reimpregnation via RTM or CVD/CVI. Column 5, lines 1-6. EP 1 731 292 A2 (Simpson et al.) discloses carbon fiber preform densification by pitch infiltration followed by RTM and a single cycle of CVD.
While densification of C—C composites with high char-yield mesophase (synthetic, coal tar pitch, or petroleum derived) pitches and CVI/CVD provide good final properties and friction and wear performance the production costs of both processes are high. The capital costs of CVD are high, while the raw materials cost of and capital costs associated with densification processes using Resin Transfer Molding (RTM) with are high char yielding mesophase pitches are also high. While the use of low and medium char yielding isotropic pitches (synthetic, coal tar and petroleum derived) combined with Vacuum Pressure Impregnation (VPI) densification processes is attractive from a cost and capital expenditure aspect the resulting C—C composites suffer from high variation in resultant microstructures (optical texture) and the lack of ability to control the microstructure during processing. This lack of uniformity of pitch matrix microstructure leads to variation in the properties and friction and wear performance of the final composite.
Therefore, one of the problems with C—C composites made from isotropic low and medium char yield pitch matrices is related to the high lot-to-lot variability in the friction performance of carbon-carbon composite brake discs made by heretofore known methods.